The present invention relates to a signal transmission system, and more in particular to a system for transmitting an ignition signal in which an ignition timing correction amount is computed from various sensor signals on the basis of a basic ignition timing determined by a pickup signal of a pickup contained in a distributor.
A block diagram of a conventional ignition timing control system is shown in FIG. 1. In FIG. 1, an ignitor 12 shapes the waveform of a pickup signal 21 produced from a distributor 11, thereby determining a basic ignition timing and an energization start timing of a coil 13. Further, the ignitor 12 determines an actual ignition timing on the basis of the basic ignition timing in accordance with retardation or advance data contained in an ignition timing correction voltage signal produced from a computer 14, and applies an ignition signal 23 indicating the energization start timing and the actual ignition timing to the coil 13 on the one hand and a basic ignition signal 24 indicating the basic ignition timing to the computer 14 on the other hand. Sensors 15 detect by means of vibration, sound, pressure or light, the engine operating conditions including engine speed, acceleration or deceleration, water temperature of the engine, knocking in the cylinders and crank position of the piston and produce sensor signals 25.
In response to the basic ignition signal 24 and various sensor signals 25, the computer 14 computes the ignition timing correction amount with respect to the basic ignition timing and supplies a voltage signal 22 corresponding to the correction amount to the ignitor 12. The coil 13 generates a high voltage at the ignition timing determined by the ignition signal 23 produced from the ignitor 12 thereby to ignite a spark plug (not shown). In the above-mentioned construction in which the ignition timing correction signal 22 to the ignitor 12 is produced in the form of a voltage signal corresponding to the correction amount, the disadvantage mentioned below results.
In the case where a signal line (designated also by 22) connecting the computer 14 to the ignitor 12 is so long that a line drop causes a potential difference between the ground potential of the ignitor 12 and the ground potential of the computer 14, the ignitor 12 reads a voltage value of the voltage signal 22 on the signal line 22 with reference to the ground potential of the ignitor 12 which is slightly different from that of the computer 14, and computes a correction value for the basic ignition timing even though the computer 14 produces a voltage value corresponding to the ignition timing correction amount on the basis of the ground potential of the computer 14 and applies it to the ignitor 12 through the signal line 22. As a result, the difference between the ground potential of the computer 14 and the ground potential of the ignitor 12 directly causes an error of the ignition timing, thereby making it impossible to accurately control the ignition timing.